Bits for drilling rocks and tip material of dressers for repairing grinding wheels have been made of diamond which has the highest hardness of all materials. Diamonds used in these tools must be approximately 1 mm or more in size. Natural diamond called "boart" is used at present since synthetic diamonds having such a size have not been manufactured on an industrial scale.
The resistance to wear of a diamond varies to a great extent depending upon the orientation of crystals and it is essential in the use of these tools to select a proper orientation. On the other hand, natural diamonds are scarcely idiomorphic because of their being subjected to dissolution in the process of growth. Generally, natural diamonds have been rounded, and have a variety of shapes depending upon the degree of dissolution. As a result, it requires considerable skill to determine the proper orientation of crystals which can serve as tools.